Early battery chargers have comprised a voltage converter circuit that converts alternating current obtained from the commercial power lines to direct current which is applied, at a controlled rate, to a discharged battery to charge the battery to "full charge." To then maintain the battery fully charged, the magnitude of charging current is reduced to a "trickle charge" level which is just enough to compensate for charge loss during battery storage. Typical values for charging current and trickle current in amperes are, respectively, 10 percent and 1 percent of the numerical battery rating in ampere-hours.
To automatically switch between high rate charging and trickle charging modes of operation, later developed battery chargers have been provided with voltage monitors that monitor battery voltage and control a semiconductor pass device, such as a power transistor or silicon-controlled rectifier (SCR) to supply high rate charging current to the battery when the battery is at less than full charge and to thereafter trickle charge the battery to maintain the battery at full charge. Examples of multiple rate battery chargers of this type are shown in Coleman et al. U.S. Pat. Nos. 3,919,618 and Seike 3,900,784.
To prevent high repetition rate cycling of the charger between high and low charging rate levels, hysteresis is typically incorporated into the control strategy of the battery charger. The term "hysteresis" refers to a two-level switching characteristic, wherein the battery voltage at which high charging rate current is terminated when battery voltage is rising is higher than the voltage at which the high charging rate current is resumed when battery voltage is decreasing. Hysteresis is typically established by providing a non-linear element as a threshold reference voltage, as in the Coleman et al patent, or by providing positive feedback within the battery voltage monitor, as in the Seike patent.
To indicate to the user the state of charge of a battery, it is helpful to display the charging mode of the operation of the battery charger, i.e., whether the battery is receiving high charging rate current or trickle charging current. Such a display circuit is disclosed in Lesher U.S. Pat. No. 3,553,561 wherein incandescent lamps are energized depending upon charging mode.
In applications wherein battery charging is to be performed at different ambient temperatures, temperature compensation networks must be provided in the battery charger circuitry to make the charging characteristics temperature independent. A problem is designing such temperature compensation networks is that charge and discharge characteristics of a typical storage (rechargable) battery vary as a function of ambient temperature. Thus, relatively complex networks must be tailored to control battery charging current as a function of temperature.
Another characteristic of battery chargers in the prior art is that automatic switchover between commercial power line operation and battery operation of the load is provided depending upon whether the power line is available as a voltage source. Thus, when the power line source is available to energize a load, the battery is electrically isolated from the load. When the power line source is removed, however, the battery is automatically connected to the load as an emergency or backup energy source to energize the load. Prior art chargers of this type, however, supply either full line voltage or full battery voltage to the load; there is no sharing by the load of both power sources. During periods of brown-out, wherein the line voltage is at a voltage level less than the rated voltage level (e.g., 110-120 v.a.c.), it is desirable to supply only as much battery current to the load as is required to compensate for the decrease in current available from the commercial power source during brown-out.
To provide short circuit protection, that is, to prevent damage to the battery charger circuitry during output short circuit or excessive charging current demand by, e.g., a battery having shorted cells, chargers are typically provided with a maximum charging current limiting network. In Seike, for example, the period of a charging current controlling multivibrator is established by a transistor that responds to the voltage drop across an output resistor. As the voltage across the resistor increases, the period of the multivibrator is reduced to compensate for excessive output current demand. Various other types of current limiting networks such as current foldback have been used in prior battery chargers to control the conductivity of power transistors or SCRs that supply charging current to the battery as a function of battery voltage.
Battery chargers of the above type tend to be relatively complex in view of the multiple requirements enumerated supra. For applications in commercial, industrial or research environments wherein cost is a significant factor, circuit complexity must be minimized. Furthermore, component count must be reduced to a minimum to improve circuit reliability. At the same time, circuit performance must be maintained as high as possible to maximize battery lifetime.
Accordingly, one object of the invention is to provide a new and improved battery charger circuit having the features indicated above while minimizing complexity and component count.
Another object is to provide a new and improved hysteresis type battery charger having an output circuit that is protected against output short circuit or excessive charging current demand.
Another object is to provide a new and improved battery charger having temperature compensation circuitry that enables charging current to track battery charging and discharging characteristics as a function of ambient temperature.
Another object is to provide a new and improved battery charger having display elements to indicate battery charging mode of operation, i.e., high rate charging mode and trickle charging mode operation.
Another object is to provide a new and improved battery charger having a display circuit that indicates when the battery to which the charger is connected has a predetermined remaining lifetime, e.g., one hour of operation.
Another object is to provide a new and improved battery charger having a graduated output transition between commercial line operation and battery operation.